1. Field of the Invention
This invention relates to an improved construction for vertical and horizontal axis wind turbine blades.
2. Prior Art
Recent emphasis on energy independence, economics and the effects of climate change has led to a re-thinking of the rate of conversion to alternative fuel supplied vehicles. Nearly all major auto makers presently have or are planning hybrid, plug-in hybrid and all electric vehicles in addition to expanding lines of natural gas fueled and alcohol fueled cars into the US from established markets elsewhere.
Better Place, a firm with a number of international and domestic electric car charging/parking lot installations, utilizes alternating current supply posts put in as branch circuits to accomplish a goal of supplying purchased ‘green electricity’ generated remotely from the site to the vehicles. Sources of economical green electricity in proximity to points of use are extremely rare.
Very large, three blade horizontal axis turbines (HAWT) are the central hope for use in supplying pollution free electrical demand to meet the needs of the national distribution grid. But they require a massive thickness of expensive composite materials at the blade root and roughly 600 man-hours of labor for each blade.
They are not economical in areas with moderate winds because of the cost elements cited above, the cost of the heavy nacelle assembly and its structural support, costs of the grid interface and the mechanism for directing the turbine into the wind. As manufacturers have steadily increased the size of the turbines and built more of them, cost per kilowatt hour has gone up . . . not down.
Rather than addressing the obvious limitations of HAWT, many are recommending transcontinental transmission from high wind areas to high population areas to meet growing energy needs. One drawback of this approach was illustrated within the report on the August 2003 power outage: Electricity purchased from utilities outside of service areas grew from 18% of total use in 1989 to about 40% of total use in 2002. Moving enough electricity across the country to both meet existing needs and electric vehicle needs from wind sites in the Great Plains area will require very expensive high voltage transmission lines and corridors. Writing off functioning coal fired power plants before they are obsolete is beyond the economic capabilities of the country.
Every kilowatt hour (kWh) of energy delivered to an end user, requires of 3.23 kilowatt hours of coal energy at a power plant. As stated by the Department of Energy, ‘energy security’ is best provided by distributed energy sources. Therefore, the use of wind energy in distributed power generation in many applications including replacement of fossil fuels has emerged as an important new option. Hartman (U.S. Pat. No. 7,329,099, 2008) shows a vertical axis design for generating heat to displace natural gas in HVAC systems and to cut coal-based electrical power emissions in existing power plants with nearby off-shore wind.
A number of earlier inventions for vertical axis turbines obtained good efficiency and self-starting capability through pivoting blades to optimize lift throughout the rotational cycle. This permitted lower costs through reducing materials usage relative to horizontal turbines. The mechanical complexity of the pitch control, however, may have been a factor contributing to the displacement of vertical turbines by horizontal turbines over the past two decades.
Sicard (U.S. Pat. No. 4,048,947, 1979) used a combination of counterweights and aerodynamic forces to orient blades to minimize drag around the circuit of rotation of a vertical turbine. Blades illustrated by Sicard are simple pipes to ease the mechanical requirements of the pivoting motion with trailing edges bonded to the pipe sections to form an airfoil.
Drees (U.S. Pat. No. 4,180,367, 1979) achieved self-starting characteristics in the ‘Cycloturbine’ by imposing an orientation at the retreating blade position perpendicular to the ambient wind direction at low starting speeds. He had an orientation parallel to prevailing wind at operational wind speeds. Mechanical actuation of the system was by cam and pushrods to each blade . . . not a significant improvement on the internal combustion engine in terms of simplicity.
Liljegren (U.S. Pat. No. 4,430,044, 1984) utilized similar cams and pushrods to control the pitch of the blades of a vertical axis turbine during the rotational cycle. This system differs from Drees in orienting both the blade positions approaching and receding from the prevailing wind roughly parallel to the tangent of the rotational circle to limit drag; Aiming for lift-based power throughout the rotational cycle and a wider range of operational speeds of the machine.
Given that improvements in vertical turbine performance can be achieved with small amounts of pitch variation, (Thesis, Pawsey, 2002), it is likely that complex mechanical drive mechanisms for pitch control used in these earlier inventions could be supplanted by simpler alternatives.
Vertical axis designs using drag based impellers have emerged to supply small amounts of site generated electricity in buildings. Naskali (U.S. Pat. No. 7,344,353, 2008) and Rahai (U.S. Pat. No. 7,393,177, 2008) are two examples of improvements on the earlier Savonius style. While effective, the complex shapes and large chords of these reactive surfaces limit the scale of the systems and increase unit electricity costs due to the complex forms.
While the approach to the orientation of the approaching and receding blades seen in Liljegren is appropriate for vertical turbines with two or three blades and low solidity, it is based on the assumption that the prevailing wind is the same as the wind direction moving around and through a vertical turbine. Studies of airflow around cylinders and consideration of the Magnus effect show that this assumption may be inadequate to capture the flow field of a vertical turbine, particularly at high solidity and/or multiple blades.
Roberts (U.S. Pat. No. 7,329,965, 2008) recognizes the importance of considering flow through the turbine assembly in his design for an “Aerodynamic hybrid” vertical turbine; but is also limited by the size and fabrication complexity factors discussed above for drag type turbines.
FloWind Inc. in conjunction with Sandia Labs conducted experiments in the late 1980s/early 1990s to reduce cost and improve performance in Darrieus style vertical turbines used in early utility installations by replacing extruded aluminum blades with composite pultrusions, (SAND 96-2205, 1996). While reasons are unclear; the newer, more elongated turbine rotor design and Sandia blade aerodynamics did not result in significantly higher efficiency or any reduced cost.
Wallaces pultrusion (U.S. Pat. No. 5,499,904 to FloWind) was large and complex, with a chord of 27 inches and four cavities in the profile separated by web portions. Production of the system using the pultrusion process was likely difficult. The field bending of the 158 ft long turbine blades into a troposkein curve was also a limitation on practicality.
Hartman (U.S. Pat. No. 7,329,099, 2008) produced a dome structure based on straight blades used as dome struts with an initial approach to variable pitch throughout the rotation. The two cavity pultrusion was simpler than that of Wallace, but there remain some issues with the design of the blade—hub attachment system and the need for simple, adaptable blade pitch control.
The new emphasis on distributed power opens up a number of new wind applications; such as local recharging of hybrid or all-electric vehicles and mid-scale wind power generation at industrial/commercial buildings, if significant cost reduction over HAWT electrical generation and drag-based, complex shape, vertical axis units could be demonstrated.